This invention relates to a dry film photoresist composition used in the fabrication of a printed circuit board, and particularly to a composition for forming a photoresist film which is developable in an aqueous solution of alkali.
Aqueous alkali developable dry film photoresist have become widely accepted in the fabrication of printed circuit boards due to its lower environmental contamination in comparison with the organic solvent developing counterpart. A typical process using a negative working aqueous alkali developable photoresist in the manufacture of a printed circuit board includes laminating a photoresist film onto a copper-clad substrate, exposing the photoresist film to ultraviolet light through an artwork or a layer formed of an image-bearing transparency, removing the unexposed part of the photoresist layer by means of a weak aqueous alkali solution, leaving the exposed part of the layer or a photoresist image on the copper clad substrate, etching the exposed part of the copper, thus leaving the part of the copper which is covered by the photoresist image, and removing the photoresist image from the substrate by a relatively stronger aqueous alkali solution.
A dry film photoresist generally consists of three layers. The first layer is a transparent polyester protective layer having a thickness of about 25 .mu.m, the second layer is a light-sensitive layer having a thickness of about 25-75 .mu.m, and the third layer is usually a polyethylene film with a thickness of about 25 .mu.m. It is known that a negative working aqueous akali developable dry film photoresist consists primarily of a carboxyl-containing polymeric binder, a monomer or an oligomer having ethylenically unsaturated functional groups, an initiator which produces free radicals upon exposure to actinic radiation, and additives such as dyes, plasticizers, and adhesion promoter etc.
Carboxyl group-containing binders are usually composed of acrylic polymers such as copolymers of methacrylic acid or acrylic acid. Examples of these polymers are disclosed in U.S. Pat. Nos. 4,535,052, and 4,234,675. Basically, a binder should be soluble in an aqueous alkali solution, and be able to provide the photoresist composition with fast dissolution properties in the developing stage.
It is generally agreed that adhesion of the photoresist pattern to the copper surface is important, particularly when fine line patterning is involved. A small amount of adhesion promoting agent was therefore incorporated in the photoresist composition. Typical examples are silane compounds, as disclosed in U.S. Pat. No. 3,758,306, and 2-mercaptobenzothiazole as disclosed in U.S. Pat. Nos. 3,758,306, 4,339,527 and 4,230,790.
However, due to the strong coupling reaction between the silane compound and the copper surface, difficulties in complete wash off of the unexposed photoresist result, leaving photoresist residue or scum on the copper surface and causing problems in etching and plating.
The 2-mercaptobenzothiazole, which is a thiol containing compound, also has a moderate reactivity with the metal copper surface, and often leaves a thin film of residual materials on the copper surface in the non-exposed area after pattern developing. Therefore, a problem similar to that of the silane compound is encountered. Furthermore, 2-mercaptobenzothiazole react with copper to produce a complex compound, as evidenced by the reddish color on the copper substrate, might cause contamination of the etching and plating solution.
As a matter of fact, the inclusion of a small amount of a thiol containing compound is known to accelerate photo-initiated free radical propagation through chain transfer reactions, and hence, is useful in photosensitive composition. There are many thiol compounds containing photosensitive compositions as taught in many literatures and patents. These compositions are exposed to ultra violet light, sometimes in conjunction with thermal energy, would induce an addition reaction between the thiol groups and ethylenically unsaturated double bonds. Generally, a polythiol and a polyene which undergo the addition reaction have substantially equivalent molar functionalities and can be cured with or without photoinitiators. Typical examples can be found in U.S. Pat. Nos. 3,976,553, 3,966,794, 3,908,039, 4,008,341, 4,230,740, 3,843,572, 3,904,499, 3,936,530 and 3,660,088. These photo curable compositions are mainly useful for molding articles, coatings, adhesives and sealants, and only a few of them are used as orgainic solvent developable photoresists. The polythiol employed predominantly are the commercially available pentaerythritol tetra(mercaptoacetate), pentaerythritol tetrakis(.beta.-mercaptopropionate) or trimethylolpropane tris (3-mercaptopropionate).
The above described polyfunctional thiol compounds have relatively high molecular weights such that the level of the typical mercaptan odor can be kept in the acceptable range as large amount is used in this kind of compositions. These polythiols are different from the thiol compounds disclosed in this instant invention, and neither of them are suitable for use in this instant invention.
Generally, polythiols contained in known photoresist films are not soluble in a weak alkaline aqueous solution. It is found that, when these insoluble polythiols are used in an aqueous developable photoresist composition, they produce scum on the area of the copper image. The scum left on the copper surface and the etching solution during the etching process, adversely affects the quality of the wiring of the circuit printed on the substrate.
The polyenes employed in the above mentioned patents predominantly carry allyl and vinyl functionalities. The polyenes with acrylic functionalities are not suitable for the compositions mentioned in the above patents due to their relatively high reactivity towards polythiol, which cause the problem in stability, particularly for the liquid composition with high content of polythiol as in most of the compositions disclosed in the above mentioned patents.